The present invention relates to a non-linear resistor comprising a sintered body of a ceramic composition which comprises zinc oxide, a specific rare earth oxide and manganese oxide which has high n-value of non-linearity based on the sintered body itself.
The conventional non-linear resistors (hereinafter referring to as varistor) include silicon carbide varistors and silicon p-n junction diodes. Recently, varistors comprising a main component of zinc oxide and an additive have been proposed.
The voltage-ampere characteristic of a varistor is usually shown by the equation ##EQU1## WHEREIN V designates a voltage applied to the varistor and I designates a current passed through the varistor and C designates a constant corresponding to the voltage when the current is passed. The exponent n can be given by the equation ##EQU2## wherein V.sub.1 and V.sub.2 respectively designate voltages under passing the current I.sub.1 or I.sub.2.
A resistor having n = 1 is an ohmic resistor and the non-linearity is superior when the n-value is higher. It is usual that n-value is desirable as high as possible.
The optimum C-value is dependent upon the uses of the varistor and it is preferable to obtain a sintered body of a ceramic composition which can easily give a wide range of the C-value.
The silicon carbide varistors can be obtained by sintering silicon carbide powder with a ceramic binding material.
The non-linearity of the silicon carbide varistor is based on voltage dependency of contact resistance between silicon carbide grains. Accordingly, the C-value of the varistor can be controlled by varying a thickness in the direction of the current passed through the varistor. However, the non-linear exponent n is relatively low as 3 to 7.
Moreover, it is necessary to sinter it in a non-oxidizing atmosphere. On the other hand, the non-linearity of the silicon p-n junction diode is caused by the p-n junction of silicon whereby it is impossible to control the C-value in a wide range.
Varistors comprising a sintered body of ceramic composition comprising a main component of zinc oxide and the other additive of Bi, Sb, Mn, Co or Cr, have been developed.
The non-linearity of said varistors is based on the sintered body itself and is remarkably high advantageously.
On the other hand, a volatile component which is vaporizable at high temperature required for sintering the mixture for the varistor, such as bismuth is included whereby it is hard to sinter the mixture to form varistors having the same characteristics in mass production without substantial loss.